Effect of Bluffbody and Circular Disc Combination on Unconfined Non-Reacting Swirling Circular Jet

This paper documents an experimental study of the combined effect of a circular disc (diameter 50 mm, 19 mm thick) placed at the end of a bluff body on the mean flow and turbulent characteristics of a turbulent swirling free jet of air issuing into still air surroundings. This paper is a part of a research program aimed at providing an improved understanding of swirling flows over a rotating cylindrical disc which could rotate at a speed of 12000 rpm. Flow pattern of the swirl jet is studied both for rotation and no-rotation cases of the cylindrical disc in the near vicinity of the jet-exit for three different inlet high Reynolds numbers viz 33800, 67680 and 101500 (only for which the recirculation effect is beneficial), critical swirl angle (30 degree-similar to what is employed in industrial furnaces and burners), and a constant blockage ratio (0.25) of the circular disc at eight different axial positions (15, 25, 40, 55, 70, 90,125 and 155mm) in the recirculation zone. A DANTEC Dynamics make constant temperature anemometer with X-probe has been used to measure mean velocities in axial and tangential directions and Reynolds stresses. A Five-hole pitot pressure probe with a digital manometer was used to measure the static pressure and estimate the points of flow reversals. For the no-rotation case, axial velocity profiles show the existence of flow reversal region behind the circular disc. It is observed that the magnitude of axial velocity, tangential velocity and the recirculation length increases with increase of Reynolds number. Mean total and reversed air flow rates are calculated by integrating the mean axial velocity profiles. In the setup used in this study and up to the axial positions investigated, the reversed flow rate as a percent of the total flow rate seems to be linearly proportional to the reversed flow zone area, being independent of the Reynolds number at a fixed nominal swirl number value of 0.59. Tangential velocity profiles exhibits the presence of forced and free vortex zones. Pressure gradient increases with the Reynolds number causing higher order of inward radial shift of the location of the maximum tangential velocity. More over the Reynolds normal and shear stresses are plotted and discussed in detail. As the Reynolds number is increased, all the turbulent stresses increase monotonically at almost all the radial position except sufficiently away from and near the axis of the cylindrical disc. Further, the results of the rotation case is compared and discussed with the no-rotation case wherever an appreciable change is noted.